feat: move LLM cache to project-local directory

- Cache now lives in <project>/.pi-project-map/cache/llm-cache.json
- Removed global ~/.cache/pi-project-map/ usage
- Cache travels with the project, no cross-project collisions
- Easy to invalidate: rm -rf .pi-project-map/cache/

Files changed:
- llm-cache.ts: accept cacheDir parameter, default to project-local
- llm-extract.ts: pass cacheDir through to cache functions
- init.ts: pass rootPath as cacheDir
- patch.ts: accept and pass cacheDir
- cli.ts: pass targetPath as cacheDir
- pi-extension.ts: pass ctx.cwd as cacheDir
- Tests updated to use temp dirs for cache isolation

src/init.ts

@@ -14,6 +14,7 @@ import type { LLMClient } from "./llm-client.js";
 export interface InitOptions {
 	verbose?: boolean;
 	llmClient?: LLMClient;
+	cacheDir?: string;
 }
 
 export async function initProject(
@@ -23,7 +24,7 @@ export async function initProject(
 	const entries = discoverProject(rootPath);
 
 	for (const entry of entries) {
-		await generateDirectoryMap(entry, options.llmClient);
+		await generateDirectoryMap(entry, options.llmClient, options.cacheDir);
 	}
 
 	if (options.verbose !== false) {
@@ -34,11 +35,12 @@ export async function initProject(
 export async function generateDirectoryMap(
 	entry: DirectoryEntry,
 	llmClient?: LLMClient,
+	cacheDir?: string,
 ): Promise<FileEntry[]> {
 	const fileData: FileEntry[] = [];
 	for (const file of entry.files) {
 		const filePath = join(entry.dirPath, file);
-		const llmData = await extractFileLLM(filePath, llmClient);
+		const llmData = await extractFileLLM(filePath, llmClient, cacheDir);
 		const astData = await extractFileAST(filePath);
 		fileData.push(mergeFileData(file, llmData, astData));
 	}
@@ -47,6 +49,7 @@ export async function generateDirectoryMap(
 		entry.relativePath,
 		fileData,
 		llmClient,
+		cacheDir,
 	);
 
 	const mapData: PackageMapData = {